Add specs for IO.popen with chdir kwarg #2052

Job	Run time
RuboCop	50s
specs (ubuntu, 3.4.4)	1m 35s
specs (ubuntu, 3.3.8)	1m 39s
specs (macos, 3.2.8)	2m 42s
specs (windows, 3.3.8)	3m 20s
specs (ubuntu, 3.2.8)	1m 31s
specs (macos, 3.4.4)	1m 22s
specs (windows, 3.4.4)	2m 57s
specs (ubuntu, 3.4.4, --enable-frozen-string-literal)	1m 37s
specs (macos, 3.3.8)	2m 45s
specs (ubuntu, 3.4.4, --parser=parse.y)	1m 43s
specs (windows, 3.2.8)	3m 28s
	25m 29s

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